There will always be an aspect of the GOAT discussion that is subjective; that's pretty much inevitable when you are comparing QB's across eras, rules changes and competition levels. There will always be honest discussion about the weight that should be assigned to playing in the Cap and FA era without a veteran and established surrounding HOF cast vs. playing with receivers who aren't routinely mugged as they run down field and with enlightened rules that protect all players, but especially QB's and "Defenseless Receivers."
Who knows what a Bart Starr or Johnny Unitas or Joe Montana would have accomplished playing under today's rules? Who knows what Tom Brady would have done if he could have played catch with Dwight Clark and Jerry Rice (plus Russ Francis) year in and year out, with Roger Craig coming out of the backfield, not to mention a consistently great O-line and D, all kept together year-after-year by ****ensian labor rules?
What's most important to me is the Tom Brady is now forever in the discussion as one of a very few players who could be considered for the Greatest of All Time, along with Baugh, Graham, Montana, Starr and Unitas (that's my list; you might add or subtract depending on your perspective).
Championships comprise the one constant that crosses all eras; beating the best of your day in one or two games when everything is on the line. Sure, competition levels are elevated today, but teams still had to play and win with the conditioning, players and equipment of their day with only one team coming out on top every year; anyone who doesn't think that those championships were hard-fought and well-earned, I suggest that you watch some vintage tape before you try to argue that with me.
In the NFL era, Starr has five championships, including two SB's; Montana has four, all SB's.*
So, as a Homer, a fourth title in the cap and FA era, along with stats that just get better every year, would put Brady into a class by himself beyond Montana. It's difficult to rule out someone as great as Bart Starr, but I'm going to lean towards the magnitude of four League Championships in this era as trumping the achievements of that true Great. Packer historians will clearly disagree with me, but that's the beauty of this discussion, isn't it?
*Bradshaw, with four SB wins, is usually not included in these discussions because of less compelling stats, the credit given to the "Iron Curtain" and an early retirement (age 34). Otto Graham is considered to have played in the NFL's definition of the "modern era," but is more difficult to compare in this context; he delivered seven league championships, three in the NFL but four in the NFL's predecessor, the AAFC. Baugh's ten league and division titles are difficult to compare and came before Canton's definition of the "modern era".